Detectors, especially synchronous detectors, have traditionally been implemented with analog multipliers or switching circuits to demodulate the signal to be detected. Using analog multipliers to detect high frequency signals, e.g., signals several hundreds kiloHertz in frequency, is restricted by limited accuracy or linearity and excessive expense. In addition, these devices have typically been unforgiving with respect to the purity of the demodulating, or detection, signal used. For example, if a square wave is used as the detection signal with a typical analog multiplier, the transient response of the analog multiplier to the detection signal limits the accuracy of the detector. And if an analog filter is used to eliminate the higher frequency components of a square wave to convert it into more nearly a sinusoidal signal, phase errors due to temperature and temporal drift in the filter itself are introduced.
To obviate such disadvantages of prior art detection schemes using analog multipliers, a multiplying digital-to-analog converter (DAC) is used instead of an analog multiplier for detection. The multiplying DAC accepts an analog input for demodulation or detection and a signal in the form of a repeating string of digital values as the demodulating input.